1. Field of the Invention
The present invention relates to a bidirectional active power conditioner. Particularly, the present invention relates to the bidirectional active power conditioner including a bidirectional DC/DC power converter and a DC/AC inverter connected thereto. More particularly, the present invention relates to the bidirectional active power conditioner using a high-frequency pulse-width-modulation (PWM) switching and controlling the bidirectional DC/DC power converter to generate a predetermined DC voltage or DC current and further using a low-frequency DC/AC inverter to convert the DC voltage or DC current into a predetermined AC voltage or AC current, so as to enhance the power quality and to improve the power characteristics of AC loads.
2. Description of the Related Art
Generally, active power conditioners are widely used to enhance the power quality and to improve the power characteristics of AC loads, For example, a conventional active power conditioner has a DC side to connect with a battery unit so as to construct an off-line Uninterruptible Power Supply (UPS). When the utility power is normal, utility power can pass through a bypass switch including in the active power conditioner for directly supplying to a load. In addition to this, the utility power can charge the battery unit included in the off-line UPS at the same time. When the utility power is abnormal or fails, a DC power of the battery unit of the off-line UPS is converted into AC power and is supplied to the predetermined load. Accordingly, no power failure will affect the operation of the load.
However, the conventional off-line UPS usually supplies an output voltage with a square wave which may affect or damage some loads such as transformers. When the off-line UPS supplying a square wave output voltage connects with a power-supply load having the function of power factor correction, the square wave output voltage might result in the over-current-protection operation of the off-line UPS.
In view of this, several off line UPS apparatuses are designed to supply a sinusoidal wave to the output voltage. U.S. Pat. No. 5,625,539, entitled “Method and Apparatus for Controlling a DC to AC Inverter System by a Plurality of Pulse Width Modulation Train”, discloses an off-line UPS apparatus which allows directly supplying a utility power to a load when the utility power is normal. Conversely, when the utility power is abnormal, the off line UPS apparatus is actuated to control a unidirectional DC/DC power converter with high-frequency PWM switching to convert a low DC voltage of a battery unit into a relatively high DC voltage which is charged into a high-capacitance DC capacitor. A full-bridge DC/AC inverter is utilized to convert the relatively high DC voltage of the high-capacitance DC capacitor into a high-quality AC voltage to supply to the load. A battery charger is further utilized to charge the battery unit when the utility power is normal. Accordingly, a control circuit applied in such a structure of the UPS is sophisticated. In addition to this, the two-stage power converters with high-frequency PWM switching can result in a low efficiency. However, there is a need for providing the high-capacitance DC capacitor to act as an energy buffer between two-stage power converters. Disadvantageously, this results in an increase of manufacturing cost.
Another off-line UPS is disclosed in U.S. Pat. No. 6,094,363, entitled “Uninterruptible Power Supply with Sine Wave Output and Energy Recycle Function”. The off-line UPS allows directly supplying utility power to a load when the utility power is normal. Conversely, when the utility power is abnormal or fails, the off line UPS is actuated to control a unidirectional DC/DC power converter to boost a low DC voltage of a battery unit into a full-wave rectified sinusoidal voltage. A low-frequency DC/AC inverter is utilized to convert the full-wave rectified sinusoidal voltage into a sinusoidal voltage to supply to the load. An energy recovery battery charger is further utilized. The battery charger with the function of energy recovery is operated to charge the battery unit when the utility power is normal or is operated to recover the redundant energy in the power conversion process to the battery unit when the utility power is abnormal. In this case, there is no need for providing a high-capacitance DC capacitor to act as an energy buffer between two-stage power converters, and only one converter is operated in high-frequency switching. Although such an off line UPS is successful in simplifying the entire structure and increasing the efficiency, an additional battery charger with the function of energy recovery is necessarily provided for charging the battery unit and recovering the redundant power in the power conversion process.
In another case, the DC side of the conventional active power conditioner connects with a solar cell unit or other renewable energy source so the active power conditioner acts as a power conversion interface of the solar cell unit or other renewable energy source for supplying AC power in a distribution power system or an AC load. The conventional power conversion interface includes two-stage power converters (i.e. DC/DC power converter and DC/AC inverter).
For example, U.S. Patent No. 6,914,418, entitled “Multi-Mode Renewable Power Converter System”, discloses a DC/DC power converter with high-frequency PWM switching utilized to convert a low DC voltage generated from a solar cell unit to be a relatively high DC voltage to which a high-capacitance DC capacitor is charged. A DC/AC inverter with high-frequency PWM switching is utilized to convert the relatively high DC voltage of the high-capacitance DC capacitor into high-quality AC power to supply to a load or to inject into a distribution power system. However, two-stage power converters acting as a power conversion interface for the solar cell unit must be operated in high-frequency PWM switching which can result in a low efficiency. In addition to this, such a structure of the bidirectional active power conditioner is sophisticated due to the fact that a high-capacitance DC capacitor acts as an energy buffer between the two-stage power converters. Disadvantageously, this results in an increase of manufacturing cost.
As is described in greater detail below, the present invention discloses a bidirectional active power conditioner which includes a bidirectional DC/DC power converter and a DC/AC inverter. The bidirectional DC/DC power converter is controlled via high-frequency PWM switching so as to generate a predetermined DC voltage or DC current while the DC/AC inverter is controlled in low frequency switching to convert the predetermined DC voltage or DC current into a predetermined AC voltage or AC current. Accordingly, only one power converter is operated in high-frequency switching so as to increase the efficiency of the bidirectional active power conditioner in such a way as to mitigate and overcome the above problem. Advantageously, the bidirectional active power conditioner is successful in reducing manufacturing cost, simplifying the control circuit and increasing the efficiency.